Split up addRandomPattern function

lib/src/Simulation.dart

@@ -9,6 +9,10 @@ class Simulation {
   Grid<bool> map;
   Grid<bool> _snapshot;
 
+  final int _RANDOM_PATTERN_AMOUNT = 20;
+  final int _RANDOM_PATTERN_DISPERSAL = 10;
+  final double _RANDOM_PATTERN_SPREAD_FROM_CENTER = 1 / 3;
+
   RuleSet rules = GameOfLife();
 
   bool dirty = true;
@@ -16,7 +20,6 @@ class Simulation {
   bool _renderEdges = true;
   bool get renderEdges => _renderEdges;
 
-  int _amount;
   int _dispersal;
 
   math.Point get gridSize => math.Point<int>(map.width, map.height);
@@ -38,25 +41,41 @@ class Simulation {
     return map;
   }
 
-  void addRandomPattern({int amount, int dispersal}) {
-    int _startingSeed = DateTime.now().millisecondsSinceEpoch;
-    math.Random rng = new math.Random(_startingSeed);
-    _amount = amount ?? rng.nextInt(20);
-    _dispersal = dispersal ?? 10;
-    int cx = rng.nextInt(map.width ~/ 3) + (map.width ~/ 3);
-    int cy = rng.nextInt(map.height ~/ 3) + (map.height ~/ 3);
+  void addRandomPattern(
+      {int seed, int amount, int dispersal, num spreadFromCenter}) {
+    math.Random rng = _getRNG(seed ?? DateTime.now().millisecondsSinceEpoch);
+    amount ??= rng.nextInt(_RANDOM_PATTERN_AMOUNT);
+    dispersal ??= _RANDOM_PATTERN_DISPERSAL;
+    spreadFromCenter ??= _RANDOM_PATTERN_SPREAD_FROM_CENTER;
 
     int sanityCheck = 0;
-    for (var i = 0; i < (_amount); i++) {
+    Map<int, bool> changeSet = {};
+    for (var i = 0; i < (amount); i++) {
       sanityCheck++;
-      getCellState(cx, cy)
+      math.Point cell = _getRandomPoint(rng, gridSize, spreadFromCenter);
+      getCellState(cell.x, cell.y)
           ? i--
-          : setCellState(
-              cx + rng.nextInt(_dispersal), cy + rng.nextInt(_dispersal), true);
+          : changeSet[map.toIndex(cell.x, cell.y)] = true;
       if (sanityCheck > 100 && sanityCheck > i * 3) break;
     }
+    mergeStateChanges(changeSet);
+  }
 
-    dirty = true;
+  math.Random _getRNG(int seed) {
+    math.Random rng = new math.Random(seed);
+    return rng;
+  }
+
+  math.Point<int> _getRandomPoint(
+      math.Random rng, math.Point size, num spreadFromCenter) {
+    math.Point absoluteSpread =
+        math.Point(map.width * spreadFromCenter, map.height * spreadFromCenter);
+    math.Point center = math.Point(map.width / 2, map.height / 2);
+    num cx = rng.nextInt(absoluteSpread.x.toInt()) +
+        (center.x - absoluteSpread.x / 2);
+    num cy = rng.nextInt(absoluteSpread.y.toInt()) +
+        (center.y - absoluteSpread.y / 2);
+    return math.Point<int>(cx.toInt(), cy.toInt());
   }
 
   void setCellState(int x, int y, bool state) {